The efficacy of antibodies (Abs) to toxins and capsules is a complex function of dose, specificity, isotype, complement activation and FcR engagement. However, 120 years after Ab-mediated immunity was first discovered, the relationships between affinity, isotype, and Fc receptor activation have not been rigorously defined for any microbial toxin or capsule. Our group has generated mAbs to five Bacillus anthracis antigens including anthrax toxin components protective antigen, lethal factor, and edema factor, capsule poly((-Dglutamic acid) (PGA), and Anthrolysin. This application proposes to investigate the structure-function relationship for these immunoglobulins with the goal of identifying the optimal combination of affinity, isotype, and specificity that determines protective efficacy. We propose to apply several state of the art technologies available to us in the form of in vitro somatic hypermutation, human FcR technology, V region expression, and Veloclmmune system to carry out the first comprehensive analysis of this fundamental immunological question while also generating new Ab reagents that are candidates for passive immune therapies. Three specific aims are proposed: Aim 1. To investigate the relationship between affinity and efficacy capacity for Abs to anthrax toxins and PGA; Aim 2. To investigate the efficacy of constant (C) regions in Abneutralization of anthrax toxins and PGA; Aim 3. To investigate the relationship between human Fc type and Ab-neutralization of anthrax toxins and PGA, We anticipate that the information generated from these studies will result in the design of more effective passive therapeutic strategies.